It is not uncommon for large business subscribers to have several directory numbers and to have a large number of attendants answering incoming telephone calls. These calls may be of different types differentiated, for example, by origination (e.g. from a private network trunk) as well as by the directory number dialed by the calling party. In modern telecommunication switches adapted for use in the so-called Integrated Services Digital Network (ISDN), the switching system accumulates data including data defining the call type for each call in progress. A subscriber may wish to have certain of the different call types served before others, in accordance with a prioritized ranking. In ISDN systems where several different call types are processed, not all servers are necessarily equipped to handle all of the call types. Thus, incoming calls must be selectively distributed to those terminals which are available to serve the particular type of incoming call.
The number of incoming calls for a particular customer frequently exceed the capability of the available servers to handle the calls. In such a case, the calls may be queued and distributed as server terminals become available. A recognized problem is the need for equitable distribution of waiting calls to servers, particularly the distribution of calls of different types on a priority basis.